The movie's final trailer reveals early on that Voldemort's beloved snake, Nagini, was once human.

We see Credence (Ezra Mileler) whisper the name Nagini before watching actress Claudia Kim turns into the iconic "Harry Potter" snake. In the books, Nagini eventually becomes one of Voldemort's horcruxes.

Kim kept the secret about playing Nagini for more than a year. When you meet her, don't expect Nagini to be as ruthless and murderous as the serpent she eventually permanently becomes.

"You've only seen her as a Horcrux," Kim told EW. "In ['Fantastic Beasts'], she's a wonderful and vulnerable woman who wants to live. She wants to stay a human being and I think that's a wonderful contrast to the character."

Nagini in human form and snake form. Warner Bros./composite by Kirsten Acuna, INSIDER

Some fans have been theorizing for quite some time that we may see Nagini in the "Fantastic Beasts" sequel this November. Rowling also confirmed Tuesday on Twitter Nagini is a Maledictus, not an Animagus, meaning Nagini has a "mysterious blood curse" which will eventually transform her into a snake permanently. The author said she's been holding onto this secret for about 20 years.

And just in case you're still making the joke that Harry Potter set Nagini free from a zoo at the start of "Harry Potter and the Sorcerer's Stone," Rowling killed that fan theory Tuesday morning.